1. Field of the Invention
The present invention relates generally to radio frequency (RF) energy attenuation. More specifically, the present invention relates to attenuating RF energy interference and RF energy emissions from an input/output (I/O) connector using an RF electromagnetic energy absorbing material.
2. Related Art
Interference received within and emissions emanating from electronic instruments, e.g., a linear amplifier, utilizing signals having high frequencies, e.g., radio frequencies (RF), is of great concern. Oftentimes, RF electromagnetic signals, hereafter referred to as "signals", within a first electronic instrument can couple with a second signal where the second signal can be internal to or external to the first electronic instrument. Such coupling can result in significant cross-talk or external interference.
The term "cross-talk" refers to the unintended electromagnetic coupling between signals travelling on wires that are in close proximity and are within the same system, e.g., signals travelling on wires within a connector. In contrast, external interference refers to electromagnetic coupling between a signal travelling on an internal wire and an external electromagnetic source. Some examples of such external electromagnetic sources are discussed below. The general term "interference" refers to cross-talk and external interference.
In the present application the term "emissions" refers to RF electromagnetic energy which emanates from a signal within a primary instrument. The primary instrument is the electronic instrument whose emissions and interference are to be minimized.
Signals which cause interference can enter the primary electronic instrument through a power-supply or through signal input/output (I/O) lines. Signals can also be magnetically coupled to closed loops in the primary instrument, or signals can be electromagnetically coupled to wires acting as small antennas for electromagnetic radiation. Any of these can be a mechanism for coupling signals from one part of a primary instrument to another.
Reducing RF interference is a significant concern when designing and manufacturing electronic instruments. It is imperative to reduce RF interference when designing electronic instruments that will be used in close proximity to other electronic instruments. This is because the primary instrument is often initially tested without other instruments in close proximity. When the primary instrument is delivered to a location where it is to be used, interference from other instruments can significantly degrade the primary instrument's performance.
Numerous techniques are used to minimize interference. These techniques rarely eliminate the interference. Instead, these techniques reduce the level of interference received by an internal signal of the primary instrument. One technique for reducing external interference involves moving the primary instrument to an environment having a lower level of external interference.
Some environments are worse than others with respect to external interference. A primary instrument that works within a desired emission range on the "bench" can perform outside of this desired emission range when placed at a different location. This is because external interference can couple with an internal signal and result in an increase in emissions from the primary instrument. Some environments to be avoided are those (a) near a radio or television station, (b) near a subway, (c) near high-voltage lines, (d) near motors and elevators, and (e) near instruments with large transformers. However, altering the operating location of the instrument is typically not a viable option. Therefore, alternate methods for reducing external interference must be implemented.
Another design consideration is the reduction of cross-talk. Cross-talk reduction can be achieved using a combination of RF line filters and transient suppressors on an AC power line. A significant attenuation from signals can be achieved using this technique. However, when the primary instrument is operating at RF, such filters can not be used without filtering out desired information which is carried on signals.
Interference within the primary instrument is a significant problem when RF coupling is involved. This problem can be particularly serious because innocent-looking parts of the instrument, e.g., wires or pins on a connector, can act as resonant circuits. Such parts can display enormous effective cross sections for RF pickup. To reduce this type of RF coupling, instrument designers attempt to keep leads short and avoid loops that can resonate. However, designing such instruments is often difficult because of practical and technical limitations.
The use of "ferrite beads" (described below) may help to reduce RF coupling. A ferrite bead is a ferrite material, i.e., a highly permeable magnetic material. The ferrite material slips onto a conductor, e.g., a wire or a pin on a connector, which is carrying signals. The ferrite material effectively acts as an RL low-pass filter. This ferrite material attenuates (chokes) RF emissions attempting to pass through it. Basically, the ferrite material alters the line inductance and provides impedance to high frequency signals such that high frequency energy does not emanate. However, this is not an ideal solution because it is difficult and expensive to manufacture a conductor such that the ferrite beads are precisely located on the wires within the primary instrument. It is also difficult to precisely determine the impedance and the inductance of a wire surrounded by a ferrite bead. As a result, it is difficult to accurately model the primary instrument's performance.
Another significant concern in the design of the primary instrument is the level of RF energy emissions emanating from the primary instrument. Governments have formed organizations which regulate the acceptable emission level for electronic instruments used within its governing territory. Examples of these organizations are the International Special Committee on Radio Interference (CISPER) in Europe and the Federal Communication Commission (FCC) in the United States. In order to satisfy the requirements set forth by such organizations, RF emissions from electronic instruments should be minimized.
Thus, what is needed is a method and a device for reducing RF emissions and RF interference at connectors which is reliable and can be precisely modelled.